In the art of testing antennas to determine their radiation patterns, and in testing radar targets for radar backscatter, the device under test is illuminated by planar waves representative of far-field radar signals from a great distance, or spherical waves representative of near-field radar signals close to the target. Both actual far-field ranges and compact ranges which simulate far-field ranges are known. However, in compact range testing, only far-zone testing for simulated large distances is known. For this far-zone testing using compact ranges, the antenna to be tested is placed in a measurement volume in which an illuminating planewave field is generated from a feed antenna or feedhorn having a fixed position at the focal point of a reflector. The test antenna is then rotated in the same way as in a far-field antenna range to obtain its far-zone antenna pattern.
It would be desirable to have a compact range apparatus that provides spherical waves necessary for near-zone measurements. Furthermore, advanced designs in antennas and radar targets require variable-zone tests to evaluate their performance parameters as the distance from radar to target changes. For example, there is presently a need for determining the performance of a tracking radar which is moving in the near-zone of its target. There is also a need for a compact range to study near-zone coupling among antennas in closely spaced antenna systems. Therefore, it would be desirable to provide a compact range apparatus which provides both near-zone measurement and continuously variable measurements from the near-zone to the far-zone.